Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching
From: Amir Goldstein <amir73il@gmail.com>
Date: 2021-01-20 12:00:13
Also in:
linux-unionfs, lkml
On Wed, Jan 20, 2021 at 12:20 PM Miklos Szeredi [off-list ref] wrote:
On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:quoted
On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng [off-list ref] wrote:quoted
The function ovl_dir_real_file() currently uses the semaphore of the inode to synchronize write to the upperfile cache field.Although the inode lock is a rw_sem it is referred to as the "inode lock" and you also left semaphore in the commit subject. No need to re-post. This can be fixed on commit.quoted
However, this function will get called by ovl_ioctl_set_flags(), which utilizes the inode semaphore too. In this case ovl_dir_real_file() will try to claim a lock that is owned by a function in its call stack, which won't get released before ovl_dir_real_file() returns. Define a dedicated semaphore for the upperfile cache, so that the deadlock won't happen. Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories") Cc: stable@vger.kernel.org # v5.10 Signed-off-by: Icenowy Zheng <icenowy@aosc.io> --- Changes in v2: - Fixed missing replacement in error handling path. Changes in v3: - Use mutex instead of semaphore. fs/overlayfs/readdir.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c index 01620ebae1bd..3980f9982f34 100644 --- a/fs/overlayfs/readdir.c +++ b/fs/overlayfs/readdir.c@@ -56,6 +56,7 @@ struct ovl_dir_file { struct list_head *cursor; struct file *realfile; struct file *upperfile; + struct mutex upperfile_mutex;That's a very specific name. This mutex protects members of struct ovl_dir_file, which could evolve into struct ovl_file one day (because no reason to cache only dir upper file), so I would go with a more generic name, but let's leave it to Miklos to decide. He could have a different idea altogether for fixing this bug.How about this (untested) patch?
Much better :)
quoted hunk ↗ jump to hunk
It's a cleanup as well as a fix, but maybe we should separate the cleanup from the fix... Thanks, Miklos --- fs/overlayfs/readdir.c | 23 +++++++---------------- 1 file changed, 7 insertions(+), 16 deletions(-)--- a/fs/overlayfs/readdir.c +++ b/fs/overlayfs/readdir.c@@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str struct ovl_dir_file *od = file->private_data; struct dentry *dentry = file->f_path.dentry; - struct file *realfile = od->realfile; + struct file *old, *realfile = od->realfile; if (!OVL_TYPE_UPPER(ovl_path_type(dentry))) return want_upper ? NULL : realfile;@@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str * Need to check if we started out being a lower dir, but got copied up */ if (!od->is_upper) { - struct inode *inode = file_inode(file); - realfile = READ_ONCE(od->upperfile); if (!realfile) { struct path upperpath; ovl_path_upper(dentry, &upperpath); realfile = ovl_dir_open_realfile(file, &upperpath); + if (IS_ERR(realfile)) + return realfile; - inode_lock(inode); - if (!od->upperfile) { - if (IS_ERR(realfile)) { - inode_unlock(inode); - return realfile; - } - smp_store_release(&od->upperfile, realfile); - } else { - /* somebody has beaten us to it */ - if (!IS_ERR(realfile)) - fput(realfile); - realfile = od->upperfile; + old = cmpxchg_release(&od->upperfile, NULL, realfile); + if (old) { + fput(realfile); + realfile = old; } - inode_unlock(inode); } }